1. Field of the Invention
The present invention relates to thermoplastic compositions. More particularly, this invention relates to compositions comprising a crystallizable thermoplastic polymer having isotropic (non-liquid crystalline) characteristics having a thermotropic (liquid crystalline in the melt) oligomer dispersed therein. These compositions have improved properties. Another aspect of this invention relates to a process for the production of such compositions, in which a particulate material is used to aid in the dispersion of the thermotropic oligomer in the thermoplastic polymer.
2. Prior Art
Crystallizable isotropic thermoplastics such as polyamides (e.g. nylon-6, nylon-66) and polyesters (e.g. poly(ethylene terephthalate), poly(butylene terephthalate) are relatively inexpensive materials and have gained wide acceptance for general molding films, textiles, yarns and industrial applications. Although most of these materials have mechanical properties suitable for general applications, further improvements to achieve superior properties, such as increased tensile strength, have not been as successful as desired.
Various additives have been proposed for addition to crystallizable isotropic thermoplastic polymers, such as polyamides (e.g. nylon-6) and polyesters (e.g. poly(ethylene terephthalate)) which are intended to improve the physical properties of fibers or films produced therefrom. Such additives include inorganic materials, such as silica which are used as fillers. The loading level of such inorganic fillers in the polymer is usually from about 10% up to about 40% by weight. The primary purpose for inclusion of these materials is to reduce cost, especially for molding parts, and the secondary purpose is to increase the rigidity and thermal stability of the molded parts. However, even though these purposes may be achieved, they are often achieved at the expense of the tensile strength and impact resistance of the polymer. Other additives include small organic compounds such as plasticizers, and other polymers with which the isotropic thermoplastic is coextruded or otherwise blended. While such additives have improved some properties of such isotropic/thermoplastic polymers, such improvement in properties has often been at the expense of other properties.
Recently, a new class of polymers has been discovered which is suitable for high strength service without the need of reinforcing agents and which exhibits a general overall balance of mechanical properties substantially enhanced over previous isotropic polymers. These polymers have been described by various terms including "liquid crystalline", "thermotropic", "liquid crystal", and "anisotropic". Briefly, the polymers of this new class are thought to involve a parallel ordering of molecular chains. The state wherein the molecules are so ordered is often referred to either as the liquid crystal state or the nematic phase of the liquid crystal state. These polymers are prepared from monomers which are generally long, flat and fairly rigid along the long axis of the molecule, and have chain extending linkages that are either coaxial or parallel. Because of the ability of such materials to exhibit anistropic properties (i.e., liquid crystalline properties) in the melt, they can readily form a product having a highly-oriented molecular structure in the shear direction upon melt processing which greatly enhances the strength of the material. Illustrative of such thermotropic polymers are thermotropic polyesters as described, for example, in U.S. Pat. Nos. 4,140,846; 3,778,410; 4,067,852; 4,083,829; 3,890,256; 3,991,013; 4,066,620; 4,075,262; 4,118,372; 4,156,070; 4,130,595; and 4,161,470. Polyazomethanes which are thermotropic are described in U.S. Pat. No. 4,048,148, and thermotropic polyesteramides are described in U.S. Pat. No. 4,272,625.
Proposals have been made to blend these thermotropic materials with isotropic themoplastic polymers to improve the mechanical characteristics of the isotropic polymers. However, in some instances, the resulting blends do not exhibit improved properties but rather exhibit properties which are merely an average of the properties of the isotropic polymer and the thermotropic polymer. For example, U.S. Pat No. 4,460,735 discloses a polymer blend which can be formed into shaped articles allegedly having improved mechanical properties. The polymer blend of this patent comprises approximately 5 to approximately 75 percent by weight, based upon the total weight of the blend, of a polycarbonate and approximately 25 to approximately 95 percent by weight, based upon the total weight of the blend, of a melt-processable wholly aromatic polyester which is capable of forming an anisotropic melt phase apart from the blend. As disclosed in the patent, the properties of these blends are merely an average of the properties of the isotropic and thermotropic polymer, and the isotropic polymer appear to function primarily as a filler for the thermotropic polymer.
Similarly, U.S. Pat. No. 4,386,174 discloses a melt-processable composition comprising at least one polymer capable of forming an anisotropic melt and at least one other melt-processable polymer characterized in that the temperature range over which the polymer can form an anisotropic melt overlaps the temperature range over which the melt-processable polymer may be melt processed. The patent discloses that the melt viscosity of such composition may be very much less than that of the melt-processable polymer in the absence of the anisotropic melt-forming polymer particularly at high shear rates corresponding to those encountered during moulding and extrusion operations. Here again the properties of the blend are merely an average of the properties of the isotropic polymer and thermotropic polymer.
In other situations where attempts have been made to blend thermotropic and isotropic polymers, thermotropic polymers have proved incompatible with isotropic polymers. The resulting heterogeneous blends exhibit properties which are no better than either the thermotropic polymer or the isotropic polymer alone. For example, M. Takayanagi et al. in J. Macromol. Sci.-Phys., B17(4), pp. 591-615 (1980) report attempts to blend nylon-6 or nylon-66 with wholly aromatic polyamides such as poly-p-benzamide or their block copolymers with nylon-6 or nylon-66. The wholly aromatic polyamides used are infusible. Similarly, M. Wellman et al., Division of Coating and Plastics Preprints, American Chem. Soc., vol. 43, pp. 7893-87 (1980) report blending of rod-like polymers with similar coil-like polymers, both having monomers with fused ring structures (e.g. poly-para-phenylene benzobisthiazole and poly-2,5(6) benzimidazole). Again the rigid polymer is infusible.
When a blend or mixture is prepared from two or more ordinary, non-polymeric materials, or from a polymeric material and a non-polymeric material, a random distribution of the molecules of the components is obtained. This random distribution provides complete mixing without the formation of groups or clusters of the molecules of any one component. Such a mixture is expected to follow the "Rule of Mixtures". The Rule of Mixtures predicts the numerical values of the modulus of a blend to be the weighted average of the numerical values of the modulus of the components.
Mixtures of most chemicallly distinct polymeric materials have been found to deviate from the behavior of ordinary mixtures as characterized by the Rule of Mixtures. The sheer size of polymeric chains and varying viscosities of the polymers restrict mixing of the components and leads to the formation of domains or clusters of molecules of the individual components. It is known in literature that in polymer blends viscosities or surface tensions of polymers in the blend affect the dispersion and the particle sizes of the dispersed polymer phase. For example, if a thermotropic polymer which has a very low melt viscosity is melt blended with an isotropic polymer which has a very high viscosity, the resultant blend under microscopic examination will normally show non-uniform distribution, and agglomeration, or coagulation of the dispersed particles of the thermotropic polymers in a continuous phase of the isotropic polymer. This non-uniform blending will result in poorer properties than either of the parent polymers, i.e., either the thermotropic or the isotropic polymer. Thus, it can be said that most chemically distinct polymeric materials tend to be incompatible in mixtures, exhibiting a tendency to separate into phases. There exists a boundary between the domains of the component polymers, and articles made from polymer blends would be expected to exhibit failure at the boundary when placed under stress. In general, then, the mechanical properties of the blend are commonly reduced as compared in the properties of the individual polymers rather than enhanced. Specific properties which may be thus affected include tensile strength, tensile modulus, flexural strength, flexural modulus and impact strength.
Accordingly, there is a need for a process of blending thermotropic and isotropic polymers to form substantially homogeneous blends which have enhanced physical characteristics.